of hydrogen, unite and are
lost in each other; it is a marriage wherein the two or three become
one. In dealing with the molecules and atoms of matter we are in a world
wherein the laws of solid bodies do not apply; friction is abolished,
elasticity is perfect, and place and form play no part. We have escaped
from matter as we know it, the solid, fluid, or gaseous forms, and are
dealing with it in its fourth or ethereal estate. In breathing, the
oxygen goes into the blood, not to stay there, but to unite with and
bring away the waste of the system in the shape of carbon, and re-enter
the air again as one of the elements of carbonic-acid gas, CO_{2}. Then
the reverse process takes place in the vegetable world, the leaves
breathe this poisonous gas, release the oxygen under the chemistry of
the sun's rays, and appropriate and store up the carbon. Thus do the
animal and vegetable worlds play into each other's hands. The animal is
dependent upon the vegetable for its carbon, which it releases again,
through the life processes, as carbonic-acid gas, to be again drawn into
the cycle of vegetable life.
The act of breathing well illustrates our mysterious relations to
Nature--the cunning way in which she plays the principal part in our
lives without our knowledge. How certain we are that we draw the air
into our lungs--that we seize hold of it in some way as if it were a
continuous substance, and pull it into our bodies! Are we not also
certain that the pump sucks the water up through the pipe, and that we
suck our iced drinks through a straw? We are quite unconscious of the
fact that the weight of the superincumbent air does it all, that
breathing is only to a very limited extent a voluntary act. It is
controlled by muscular machinery, but that machinery would not act in a
vacuum. We contract the diaphragm, or the diaphragm contracts under
stimuli received through the medulla oblongata from those parts of the
body which constantly demand oxygen, and a vacuum tends to form in the
chest, which is constantly prevented by the air rushing in to fill it.
The expansive force of the air under its own weight causes the lungs to
fill, just as it causes the bellows of the blacksmith to fill when he
works the lever, and the water to rise in the pump when we force out the
air by working the handle. Another unconscious muscular effort under the
influence of nerve stimulus, and the air is forced out of the lungs,
charged with the bodily wa
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